1. Field of the Invention
The present invention relates to a device for removing common-mode noise which is introduced into a balanced modulated color signal by a switching operation of a Gilbert multiplier in a balanced chrominance modulator.
2. Description of the Related Art
FIG. 1 is a circuit diagram for a conventional balanced modulating device using a Gilbert multiplier. In FIG. 1, reference numeral 10 denotes a R-Y modulating circuit, reference numeral 20 denotes a B-Y modulating circuit, reference numeral 30 denotes a chrominance amplifying circuit, reference numerals R0-R11 denote resistors, and reference numerals Q1-Q22 denote transistors.
The R-Y modulating circuit 10 includes an active load 1 connected to a power source Vcc for supplying current and a biasing circuit 5. The R-Y modulating circuit 10 also includes a modulator 3 that receives a red chrominance signal R, a luminance signal Y, and a carrier signal I, along with its complementary signal IB, and modulates these inputs in order to produce a R-Y color difference signal. The active load 1 includes a resistor R0 connected between the power source Vcc and the collector of a transistor Q1 whose base and collector are connected to each other, i.e. Q1 is diode-connected. The modulator 3 is composed of a Gilbert multiplier which has a very small modulation gain. The biasing circuit 5 includes transistors Q8 and Q9 whose bases are commonly connected, and resistors R2 and R3 which are connected between the emitters of transistors Q8 and Q9, respectively, and a ground terminal.
The B-Y modulating circuit 20 has an active load 21 connected to the power source Vcc for supplying current and a biasing circuit 25. The B-Y modulating circuit 20 also includes a modulator circuit 23 that receives a blue chrominance signal B, the luminance signal Y, and another carrier signal Q, along with its complement QB, and modulates these input signals to produce a B-Y color difference signal. The active load 21 includes a resistor R4 connected between the power source Vcc and the collector of a diode-connected transistor Q12. The modulator 23 is composed of a Gilbert multiplier circuit which has a very small modulation gain. The biasing circuit 25 includes transistors Q17 and Q18 whose bases are commonly connected and resistors R6 and R7 which are connected between the emitters of transistors Q17 and Q18, respectively, and the ground terminal.
The R-Y color difference signal generated in the R-Y modulating circuit 10 and the B-Y color difference signal generated in the B-Y modulation circuit 20 are vector-synthesized into a pair of differential balanced-modulated color signals CC and CCB, where CCB is the complementary signal to signal CC and where the signal CC can be expressed by: EQU CC=(B-Y)+j(R-Y).
The chrominance amplifying circuit 30 receives the balanced-modulated color signal CC, amplifies it at a predetermined gain, and outputs an amplified modulated color signal OK.
However, when vector-synthesizing the R-Y and B-Y color difference signals in accordance with the above equation, noise generated during the switching operation of the transistors within modulators 3 and 23 is also input to the chrominance amplifying circuit 30 and amplified. Thus, the amplified modulated color signal OK contains amplified noise, as shown in FIG. 3C, which results in Y/C interference phenomenon. For example, when a white line is to be displayed on a screen, that line will appear to be colored.